When a sealed secondary cell or battery enters an overcharge state for some cause, conventionally, the internal pressure of the cell may rise beyond the range of a normal state. Most of secondary cells therefore have safety valves to avoid the internal pressure from excessively rising. Such a safety valve is made more fragile than other parts so that it is broken to open when the internal pressure exceeds the limit and function as a release port for releasing the gas accumulated in a cell.
Another secondary cell or battery includes a CID that works if an internal pressure is higher than an internal pressure in a normal state but lower than a valve opening pressure of a safety valve (see Patent Document 1, for example). The CID disclosed in this document includes a diaphragm 35 made of a bent metal plate. As the internal pressure of the secondary cell rises due to overcharge or other causes, the diaphragm 35 is pushed up and deformed. Upon deformation, the diaphragm 35 is separated from a connecting metal 34 connected to an electrode plate or sheet, resulting in disconnection between the diaphragm 35 and the connecting metal 34. Accordingly, a current path is interrupted at this site and thus the cell is not charged any further.